The researchers believe that measuring RBP4 would potentially be an effective way to assess body fat, and that treatment to cut levels of the protein might also have health benefits.

In previous work, they showed that cutting RBP4 levels in obese mice helped the animals to make better use of the hormone insulin - and thus reduce their risk of diabetes.

They also showed that measures to improve insulin sensitivity in human subjects resulted in a drop in RPB4 levels.

Researcher Dr Matthias Bluher said: "We believe that in the near future, measurements of RBP4 serum concentrations might serve as a novel biomarker for visceral obesity and increased risk for type 2 diabetes and other adverse outcomes of visceral obesity.

"In addition, pharmacological interventions that reduce RBP4 levels might be a new approach in the treatment of metabolic syndrome and visceral obesity."

The only known function of RBP4 is to carry vitamin A in the blood.

People with type 1 diabetes need daily injections to help them absorb glucose to use as fuel. Failure to take correct doses can lead to rapid weight loss.

Charity Diabetes UK estimates that up to one-third of young women with the disease miss injections to stay thin.

Doctors warn that the "diabulimia" eating disorder can lead to blindness, heart and kidney disease.

US doctors recently went public on their concerns about the practice.

Bell has spent years studying how human beings store and use their adipose tissue, or fat. He has carried out studies showing that people who would be considered slim can have large quantities of fat within them.

'This is particularly true of men who have a slim build but who do little or no exercise,' he said. 'We know now that 40 per cent of people have fat infiltration of the liver, which is linked to so many other health problems.'

He said of Schwartz: 'He is slim, he's not overweight, but you can see there are some areas where there is a bit of a build-up of visceral fat. He doesn't have a lot of subcutaneous fat [the kind that lies just under the skin], but I can see there is quite a bit around the organs and some in the muscle.'

Thanks to this new technique, Bell and others are able to understand why appearances can be so deceptive. Someone like Schwartz, who is young, falls into the category of those who need to start changing their lifestyle. Unknowingly, he is on the way to becoming what is jokingly described as a 'Tofi' - Thin on the Outside, Fat on the Inside. Tofis probably need to worry more about their health than others, because the fat deposits they carry are hidden in the white fat that lies around their vital organs, streaked through their underused muscles, and wrapped around the heart. It is this fat that sends out the chemical signals which eventually lead to insulin resistance, diabetes and heart conditions, rather than the fat lying in dimples underneath the skin.

What really counts, says Bell, is how and where the body's energy supply is stored. Fat cells are extremely intelligent - 'versatile players', as the American obesity specialist Roger Unger called them - which hang on stubbornly even through crash diets. For years, doctors saw fat tissue as a kind of passive storage compartment, but new research has shown that the fat cells, or adipocytes, are dynamic beings.

In Japan, sumo wrestlers have been put through MRI scanners to look at their fat composition. Even though they have a BMI of 56 and are eating up to 5,000 calories a day, they have very little internal fat. 'They have low cholesterol, they have low insulin resistance and a low level of triglycerides [fatty acids],' said Bell. 'Their fat is all stored under the skin, on the outside.'

"The rate of overweight babies has increased quite dramatically in 0- to 6-year-olds over a 20-year period," Dr. Matthew Gillman said.

Gillman was part of the study that looked at more than 120,000 children under 6 in Massachusetts.

"What was most surprising about our study is that we found the rate of among youngest children -- our infants -- was rising over this time," he said.

According to the study, overweight babies between birth to 6-months-old increased by 74 percent. Researchers said that translates into babies becoming overweight adults. It's an increase of 60 percent.